As an image forming apparatus such as a printer, a facsimile machine, a copying machine and a complex machine thereof, for example, an ink jet recording apparatus is known. The ink jet recording machine performs recording (that is synonymous with image formation, picture printing, character printing, printing, and the like) by ejecting an ink drop from a recording head onto a recording-medium such as a recording paper (referred to as a “paper” below, but the medium is not limited to a paper, and the medium can be also referred to as a recording-medium, a transcription paper, a transcription medium, recording material, or the like). The ink jet recording machine has some advantages of having the capability of recording a high-definition image at high speed, low running cost, low noise, and further, easily recording a color image using multi-color inks.
In such an ink jet recording apparatus, it is necessary to increase the precision of the landing position of an ink drop on a paper for the attainment of high image quality. Therefore, for example, it is known to prevent jams or contamination caused by the contact of a recording head with a paper by uniformly and positively charging a conveyer belt for conveying the paper so as to hold the paper due to electrostatic attraction force, to keep the distance between the recording head and the paper constant, to control the conveyance of the paper accurately to prevent the displacement of a paper, and to prevent floating of the paper, as disclosed in Japanese Laid-Open Patent Application No. 4-201469, Japanese Laid-Open Patent Application No. 9-254460, and Japanese Laid-Open Patent Application No. 2000-25249.
However, it is known that, as the conveyer belt is thus uniformly and positively charged to hold the paper due to the an attraction force, an ink drop ejected from the recording head is influenced by an electric field so that the displacement of the landing position of the ink drop on the paper is caused and ink mist flows back to the side of the recording head.
In order to prevent the displacement of the landing position of an ink drop or the flowing back of ink mist, it is known that, to the surface of the paper on a conveyer belt having a surface charged with an uniform charge, a charge with a polarity opposing that of the conveyer belt is applied at the upstream side in directions for conveying a recording head so that the electric potential of the surface of the paper is lowered and the influence of the electric field on the ejected ink drop is reduced, and the electric potential with the same polarity as that of the surface of the conveyer belt is lowered from the side of the paper so that the holding of the paper to the conveyer belt due to the attraction force is improved, as disclosed in Japanese Laid-Open Patent Application No. 2000-25249.
Further, as a method for charging a conveyer belt, it is known that an alternating charging pattern is formed by contacting a surface of the conveyer belt with a voltage application device and alternately applying a positive charge and a negative charge in a strip-shaped manner on the surface of the conveyer belt, as disclosed in Japanese Patent No. 2897960.
As described above, when a paper is held by electrostatic attraction force, an electric field is provided between a surface of the paper and the recording head. Therefore, there are problems in that an ink drop ejected from the recording head is polarized by the influence of the electric field so that the traveling of the ink drop is disturbed and thus recording cannot be performed well, and also, ink mist caused by the traveling of ink drops flows back to near or adheres to an ejection portion of the head (a nozzle face formed on the nozzle) as a result of the polarization of the ink drop.
To address these problems, charges in an alternating charging (positive charging and negative charging due to an alternate current) pattern are applied to the conveyer belt and, as a result, an attraction force is generated between the paper and the conveyer belt as disclosed in Japanese Patent No. 2897960. Simultaneously, positive charges and negative charges induced alternately on the surface of the paper are conveyed so that the influences of the positive charges and the negative charges are canceled by each other so as to reduce the average electric potential on the surface of the paper. Then, the electric field that causes the displacement of the landing position of the ink drop and the flowing back of the ink mist is reduced.
Meanwhile, the use of a pigment-containing ink, in which an organic pigment or carbon black is used, is being studied or is in use as a coloring agent in a recent image forming apparatus using ink in order to attain high quality character printing on normal paper. Since a pigment is different from a dye and has no or little solubility to water, the pigment is normally mixed with a dispersing agent and is used in aqueous ink on the condition that the pigment is stably dispersed in water through a dispersion process. Such a pigment-containing ink generally has a viscosity higher than that of a dye-containing ink and the viscosity of the pigment-containing ink drastically varies within a range of 5 mPs through 20 mPs.
A drop of such highly viscous ink is deformed into a cylindrical shape such that it instantaneously extends long in ejection directions after a main drop of ink is ejected. Then, a phenomenon of dielectric polarization occurs such that a charge on the conveyer belt induces an opposite charge on a portion of the ink drop which portion is closest to the conveyer belt and a charge further opposite thereto, that is, a charge with the same polarity as the charge on the belt on a portion of the ink drop which portion is furthest from the conveyer belt. In another moment, the dielectrically polarized ink cylinder is divided into ink at the side of the conveyer belt which become a drop shape and ink at the side of the head which returns to the inside of the nozzle. At this time, an intermediate portion of the ink cylinder is divided more finely and become tailing ink mist. Since the trailing ink mist has the same charge as the charge on the conveyer belt, the mist is repelled by the belt and adheres to and often contaminates the nozzle face.
Consequently, the problem still remains that the adhesion of ink mist to a nozzle face of a recording head cannot be eliminated by only the conventional charging control for a conveyer belt in an image forming apparatus using such a highly viscous recording liquid.